Where electrical circuit breaking devices, such as switches, etc., are located in an area in which explosive atmospheres may be present, provision must be made to prevent those circuit breaking devices from igniting the atmosphere and thereby causing an explosion. The conventional procedure is to enclose such devices in a case which will prevent any ignition which occurs within the case from propagating to the space outside the case. Such cases must, of course, have access openings so that the installer or a repairman can get at the electrical devices within the case. Such access openings are provided with covers. A common practice is to utilize an interlock between the electrical device within the case and the cover which will prevent the cover from being removed should the device be in switch-closed position. Otherwise, it would be possible for a careless person to remove the cover and thereafter move the device to switch-open position, with the result being that the flame occurring within the case could easily propagate to the outside through the access opening from which the cover had been removed. An example of such a device will be found in U.S. Pat. No. 3,743,800.
It has been found that there are occasions where it is important that a serviceman, etc., be able to remove the cover while the electrical device within the housing is in switch-closed position. For example, one or more large machines may be electrically connected to the box. It could, for example, be a machine involved in a continuous chemical process which, if interrupted, would result in a prolonged restarting operation and loss of product in process. Shutting such machines down even briefly might be an expensive procedure. Yet, for some good reason the serviceman must have access to the interior of the housing. Also, the serviceman may have gas detectors available by which he can determine that an explosive atmosphere does not exist about the housing; and, therefore, a temporary removal of the cover to provide access to the interior of the housing could be made without fear of causing an explosion. Even if he knew an explosive atmosphere existed, the experienced serviceman might know that, with his knowledge and experience, work could safely be performed inside the box without the necessity of interrupting the electrical circuit.
One solution to this problem might be to remove the safety interlock altogether. However, this would permit careless individuals, or individuals who did not have sufficient knowledge of the situation to be careful about what they did, to remove the covers. Thus the possibility of an explosion would remain. The solution provided by the present invention is to provide a defeater operable from the exterior of the housing to render the safety interlock ineffective so that the covers can be removed. The exterior, operable part of this defeater is inconspicuous. One not sufficiently knowledgeable to know about the interlock, why it was there, etc., would not recognize the defeater for what is was and therefore would not use it to obtain access to the interior of the housing. Instead, he would follow the conventional practice of turning the switch handle to the "off" position to permit him to remove the cover.
Further objects and advantages will become apparent from the following description.